The present invention relates to screen printing machines, and more particularly to an improved apparatus for off-contact screen printing.
The printing of images on articles, and particularly clothing (such as T-shirts and the like), is commonly accomplished using screen printing machines. Generally, these machines are provided with a print squeegee which is stroked across the surface of the screen so as to force printing ink through the screen thereby effecting printing. A screen printing apparatus of this general configuration is disclosed, for example, in U.S. Pat. No. 3,795,189, and in a copending U.S. Pat. No. 4,939,991, disclosures of which are hereby incorporated by reference. In operation, the clothing or other material to receive the print image are placed on pallets so that the surface to be printed upon is exposed. The pallets are then indexed to individual print stations where a printing head lowers the printing screen to position slightly above the print surface. Subsequently, flood and squeegee blades are stroked across the screen to print an image on the print surface. As the squeegee blade passes over the screen, it momentarily forces the screen to contact the printing surface while at the same time forcing ink through the screen to effect printing. Once the squeegee has passed a point along the screen, that portion of the screen lifts off the article as a result of the elasticity of the stretched screen material, thus giving rise to the term "off contact printing".
As would be understood by one skilled in the art, the distance between the screen and the printing surface is critical to the quality of the print image. This distance varies depending on the thickness of the article to be printed upon, the particular printing effect desired, and the characteristics of the screen and ink. In addition to utilizing the appropriate distance between the screen and printing surface, it is important that the screen be parallel to the surface to be printed upon so as to avoid uneven, faint or smudged print images.
In the past, the distance between the screen relative to the print surface during printing has been accomplished using a plurality of screw-type elements located at the four corners of the screen frame. Although this arrangement allowed for the accurate positioning of the screen, it was tedious and time-consuming, particularly in view of the need to keep the screen level. Moreover, it has been found that these screw-type adjustments tend to loosen under the constant vibration associated with the operation of the printing machine.